Sizing a custom bullet mold.

[flynth]

I'm making my own bullet mold using 7xxx aluminium (for the ease of machining mostly - also I like aluminium molds). This mold is a replica of a 1851 Swedish Kammerlader for Navy bullet.

It is a black powder breech loading rifle. The bullet will be cast from pure lead. The target diameter is 0.610 in (15.5mm).

Can someone who made their own molds advise me on sizing the cavity, please?

Should I simply calculate it based on thermal expansion of aluminium at mold casting temp, both expansion of the mold and contraction of lead or is there some rule of thumb for sizing cavities?

Currently I'm halfway done with the block. I have the alignment pins set, the inside mating surfaces ground and the air escape channels cut. I'll be lathe boring the cavity by holding the block in a 4 jaw chuck.

[TomAM]

Try a cavity diameter of .6135"

Tom

[flynth]

I'm after a rule I can apply to various molds I'll no doubt cut in the future. Is the. 6135 answer coming from experience of making aluminium molds, or was it calculated ?

Using thermal expansion of aluminium, contraction of lead and mold casting temp of 200c (392f) I'm getting 3 thou.

[gwpercle]

Tom's Occupation is listed as Professional Mould Maker... Tom - mould maker ...
which would probably make him the Tom of Accurate Molds fame ... TomAM ... Tom Accurate Molds ?
check them out at www.accuratemolds.com .
I may be wrong , I have put 2 and 2 together before and came up with 5 .

Tom is that You ?
Gary

[Mk42gunner]

I would take Tom's advice.

My non machinist thought was to sneak up on it by cutting to a certain diameter then trying the mold. The bad thing about my idea is having to set it up in the four jaw chuck a few times.

Robert

[TomAM]

Yup, it's me.
There might be a formula, or rule, but I don't know what it is.
I use a spreadsheet, based upon experience (extensive testing).

[flynth]

Ok, thanks. I'll make it 0.6135. Better if it ends up slightly over than under. Although if it ends up under I can always cut a little more.

I'm planning to use a 4 jaw chuck to hold the blocks. I haven't decided if I'll use my large lathe (more rigid less precise) or a Chinese mini lathe (a lot less rigid, but very precise after some Tlc)

Here are two pictures of my cutter as it is being made. I've since heat treated it and ground most of the relief. It is made from water hardening steel (so called "silver steel" - 1.2210 in EU).
https://i.ibb.co/5BG1k69/20210810-151113.jpg
https://i.ibb.co/R4SDX5Y/20210810-151128.jpg

This is a rather short and stubby bullet.

If the mold comes out well maybe I'll post a thread with more details.

Having some trouble holding aluminium blocks on my surface grinder my next mold most likely will be made of steel (or cast iron if I can get the right kind locally).

[gwpercle]

Yup, it's me.
There might be a formula, or rule, but I don't know what it is.
I use a spreadsheet, based upon experience (extensive testing).

Experience + extensive testing beats a formula ... Seven Ways To Sunday !
At least in my old school backward way of thinking .
Gary

[flynth]

Interestingly, the bullet is oversize by quite a bit (almost 2 thou) :-(

I turned a precision pin to exactly 0.6135, with the first 40 thou in length 0.6130. I also kept checking frequently while cutting. I'm pretty sure my cavity ended up bang on. The smaller bit of the pin slides in fairly easily. The target diameter is a tight hand push fit.

I cast with pure lead between 450c~470c (840f~880f) and the mold around 200c (390).

The bullet ended up 0.6118 (15.54mm). 1.8 thou over. I can live with it. I'll test it as is, and then I'll size it down and it'll be tested at 0.610.

If the sized down bullet proves more accurate I'll make the next cavity sized at 0.6117 (15.53mm).

One more thing that may have some impact on bullet diameter. I use a Lyman ladle to cast by filling it full. Then pressing the ladle spout to the sprue funnel. Raising it so the ladle "pressurises" the cavity for few seconds (4~8s depending on the mold). If I don't do it this way with large bullets I don't get crisp grease groove edges etc.